A spinning machine includes a spinning device adapted apply a twist to a fiber bundle to produce a spun yarn. The spinning machine also includes a draft device adapted to draft the fiber bundle (stretch the fiber bundle). The draft device sandwiches the fiber bundle (or the sliver) with a rotating draft roller pair and transports the fiber bundle to stretch the fiber bundle into an appropriate fiber width, and to supply the fiber bundle to the spinning device.
In this type of draft device, the draft roller rotates at high speed, and thus airflow (accompanying airflow) is generated along an outer peripheral surface of the draft roller. This accompanying airflow is known to greatly influence yarn quality. Therefore, conventionally, attempts have been made to devise a shape of the draft roller to reduce an adverse effect by the accompanying airflow. Such a draft roller is described in Japanese Unexamined Patent Publication Nos. 7-126926 (Patent Document 1), 2010-163702 (Patent Document 2), and 2005-113274 (Patent Document 3).
Patent Document 1 discloses a front top roller in which an effective roller width is narrowed to about a half or more of a standard width. In other words, a step is provided on an outer periphery of the front top roller disclosed in Patent Document 1. In Patent Document 1, according to such a configuration, the drafted fiber bundle is not influenced by the accompanying airflow and cotton fly is hardly moved to both sides of the front top roller.
In Patent Document 2, a step formed on a front top roller (referred to as gap L in Patent Document 2) is preferably greater than or equal to 1 mm and smaller than or equal to 3 mm. However, Patent Document 2 does not disclose about other specific dimensions of the step.
In this regard, according to Patent Document 3, in high speed spinning exceeding 300 m/min (experiment was conducted at specifically 350 m/min in Patent Document 3), a dimension of a step of a front roller (referred to as gap B in Patent Document 3) is suitably 1.5 mm. In Patent Document 3, drawbacks occur even if the step of the front roller is too narrow or too wide.
As described above, when forming the step on the front top roller, it is known that the step of 1.5 mm is the most suitable. As pointed out in Patent Document 3, drawbacks occur even if the dimension of the step is too high or too low. Therefore, the dimension of the step of the front top roller is not ventured to be set to other than 1.5 mm.
The front top roller of the draft device is generally made of rubber. With such a rubber roller, a portion that makes contact with the fiber bundle (central portion in an axial direction) tends to wear and become recessed through use. That is, the rubber front top roller is a consumable. However, if the front top roller is discarded with minor wear, an operation cost of the spinning machine increases. Thus, attempts have been made to abrade a surface of the worn-out front top roller into a smooth state (state in which the recess is eliminated) so that the front top roller can be reused.
Meanwhile, since the outer diameter of the front top roller becomes smaller with the abrasion of the surface of the front top roller, the step formed on the front top roller becomes smaller. As a result, yarn quality degrades. Therefore, in view of the quality of the spun yarn to be produced, a minimum diameter of the usable front top roller is set and the reuse of the front top roller, which outer diameter has become smaller than the minimum diameter, needs to be prohibited. Thus, the conventional front top roller cannot be repeatedly abraded and used for a long period of time.
Patent Document 3 assumes a spinning speed of at least 300 m/min as “high speed spinning”, but due to further improvement in the spinning speed of recent years, the spinning speed may exceed 400 m/min. Therefore, a rotation speed of the draft roller is becoming higher in recent years than at the time of the application of Patent Document 3, and an influence of the accompanying airflow on the yarn quality is also assumed to have changed. Therefore, the shape of the draft roller assumed as optimum in the above patent documents may not be optimum in the current high speed spinning (spinning speed of around 400 m/min). In other words, there is still room for improvements to improve the shape of the draft roller to enhance the yarn quality.